They're not cute and fuzzy like polar bears, and they don't provide a spectacular underwater color show like the Caribbean coral reefs. But North America's freshwater mussels may be just as vulnerable to global warming as other, more charismatic species.

New research overseen by the U.S. Geological Survey shows that juvenile mussels have difficulty surviving in higher water temperatures that may happen more frequently in North America's rivers and lakes as the planet warms.

The authors of the study, published in the journalFreshwater Science, say that because these unassuming mollusks provide important ecosystem services like water filtration, their extinction could have ripple effects in river and lake ecosystems across the country.

Freshwater mussels are already "the most imperiled group of animals in North America," said USGS biologist Teresa Newton, co-author of the study. "If we're not careful, they're going to wink out in front of our eyes."

Of the 302 mussel species native to North America, a majority are either extinct or in serious trouble. Nearly 90 are classified as endangered, and this September, two additional freshwater species were listed by the Fish and Wildlife Service (Greenwire, Sept. 26).

According to Tierra Curry, a biologist with the Center for Biological Diversity who works to secure federal protection for the animals, dams and pollution have been primarily responsible for the mussels' decline until now. Climate change, Curry said, "could be the straw that breaks the camel's back."

Because of the numerous threats already facing mussels, "it really is important to understand their thermal sensitivity," said W. Gregory Cope, a professor in North Carolina State University's Department of Environmental and Molecular Toxicology who also researches the potential impacts of climate change on the mollusks.

According to Newton, river temperatures in the southeastern United States, where many freshwater mussel species live, have already been recorded at close to 40 degrees Celsius (104 degrees Fahrenheit) under drought conditions.

To better understand how this might affect populations, Newton and her co-authors conducted laboratory tests on how the juveniles of three mussel species fared when their surrounding water temperatures were incrementally increased over 28 days.

Mussels already living at their thermal limits?

They found that temperatures between about 25 and 30 C were enough to kill at least half the mussels in their samples, which were about 2 months old and less than a millimeter in length. Moreover, for two of the species in the study, the researchers peered through microscopes to observe that the mollusks' heart rates significantly slowed as the temperatures rose.

"These data suggest that mussels are living near their upper thermal limits, if we can infer that from these laboratory studies," Newton said.

Newton has conducted other research indicating that some mussel species may cope with warmer waters by burrowing deeper into the sediments of rivers and lakes.

But because habitats for different species are already highly fragmented, she guesses that other freshwater mussels will have difficulty adapting.

"Some species are probably going to do much better with dealing with climate change than other species, but we don't quite know who those players are," she said. "If we can identify that these 10 species are exceptionally sensitive to thermal stress and maybe these 10 are not, you might expect down the road that you might see a shift in species composition."

For those species that aren't able to survive the shift to higher temperatures, the ecosystems that depend on the "voracious filter feeders," as Newton calls them, will experience repercussions.

For example, mussels help cycle nutrients in rivers, sustaining insects like mayflies and stoneflies that in turn are important food sources for freshwater fish.

According to Curry, mussels are also important food sources for muskrats, otters, fish and some reptiles.

Newton said, "If mussels indeed perish because of the effects of climate change and they perform these important services in our rivers, then that could have cascading effects on other animals that depend on mussels for their survival."